The present invention relates to electronic communications systems, and more particularly to a method and apparatus for filtering interference and nonlinear distortions in a signal communicated from a transmitter to a receiver via a communication path.
The invention is particularly suited for use in connection with a television distribution system, such as a hybrid fiber/coax (HFC) network, in which a subscriber terminal such as a set-top box or cable modem receives television and/or data signals from the distribution system "headend" via a "downstream" communication path and sends information back to the headend on an "upstream" return path. In such an environment, the interference filtered by the present invention is often referred to as "ingress" or "ingress noise" and the nonlinear distortions of concern comprise composite second order (CSO) and composite triple beat (CTB) distortions.
Hybrid fiber/coax networks, which are based on branch and tree architecture, provide a cost-effective means for delivering downstream broadband services such as analog/digital video and high-speed data. In addition, they provide subscribers with high-speed data upstream transmission, for example, in the 5-42 MHz portion of the RF spectrum. Cumulative ingress noise is the main impairment in the return-path portion of HFC networks. The types of ingress noise, which appear on the return-path, can be classified as follows:
A. Narrowband short-wave signals, originating from radio stations and other sources, coupled to the return-path cable plant at the subscriber location or in the distribution plant. PA1 B. Common mode distortion originating from non-linearities in the cable plant. PA1 C. Location specific interference generated by an electrical device at the subscriber location. See, e.g., C. A. Eldering, N. Himayat, and F. M. Gardner, "CATV Return Path Characterization for Reliable Communications", IEEE Communications 8, 62-68 (1995). The amount of cumulative ingress noise in the return-path network is essentially the limiting factor in determining the maximum number of simultaneous users and the maximum data transmission rate that can be achieved.
Video signals sent to set-top boxes are also often subject to "burst/impulse noise" originating from peak Composite Second Order (CSO) and/or Composite Triple Beat (CTB) distortions. These distortions are generally present at known frequencies, which depend upon the television frequency plan used for the analog video signals. In cable television systems, such frequency plans include the integrally-related-carrier (IRC) plan and the harmonically-related-carrier (HRC) plan. CSO and CTB distortions can lead to video blocking and visually degraded areas in a television picture. One method for addressing the CSO/CTB distortion problem in multi-channel AM-VSB (amplitude modulated vestigial sideband)/QAM (quadrature amplitude modulation) video transmission systems and the like is disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 09/170,852 filed Oct. 13, 1998 and entitled "Method and System for Enhancing Digital Video Transmission To A Set-Top Box." In the system disclosed in that patent application, the performance of hybrid analog and digital video transmission systems is improved by determining the relative magnitude and frequency locations of nonlinear distortions, identifying the analog channel frequency plan, and then selecting a digital channel map based thereon.
It would be advantageous to have a robust and cost-effective method and apparatus for filtering interference (such as ingress noise or other interference types) and nonlinear distortions in a signal communicated from a transmitter to a receiver via a communication path, such as a return path signal from a set-top box or the like. It would be further advantageous to provide such a method and apparatus that operate adaptively, so that interference (e.g., ingress) is efficiently filtered even when the frequency of the interference peaks changes over time. Such a method and apparatus should enable a plurality of interference peaks to be filtered, and should automatically adapt to changing conditions in the interference.
It would be still further advantageous to provide a method and apparatus for filtering nonlinear distortion in a signal communicated from a transmitter to a receiver via a communication path. Such a method and apparatus would be particularly useful in the downstream channel of an HFC cable television distribution system, where either an IRC or HRC frequency plan is used.
The present invention provides methods and apparatus enjoying the aforementioned and other advantages.